Operational Amplifiers Selection Guide # AD820B Differential Amplifier Technical Documentation
## 1. Application Scenarios
### Typical Use Cases
The AD820B is a precision differential amplifier designed for demanding industrial and automotive applications requiring accurate signal conditioning in harsh environments. Key use cases include:
Current Shunt Monitoring
- Motor control systems for precise current measurement
- Battery management systems monitoring charge/discharge currents
- Power supply current monitoring with high common-mode voltage rejection
- Typical configurations: ±12V to ±18V supply rails with shunt resistors from 0.1mΩ to 100mΩ
Bridge Sensor Conditioning
- Strain gauge amplification in industrial weighing systems
- Pressure transducer signal conditioning
- Load cell amplification with excellent common-mode rejection
- Applications requiring rejection of 60Hz/50Hz power line interference
Industrial Process Control
- 4-20mA current loop receivers
- Thermocouple amplification with cold junction compensation
- Isolated signal acquisition systems
### Industry Applications
Automotive Systems
- Electric vehicle motor controllers
- Battery current monitoring in hybrid/electric vehicles
- Engine control unit sensor interfaces
- Advantages: Operates in -40°C to +125°C temperature range, AEC-Q100 qualified versions available
Industrial Automation
- PLC analog input modules
- Motor drive current sensing
- Robotics position feedback systems
- Practical advantage: Withstands industrial noise environments with 80dB CMRR at 60Hz
Medical Equipment
- Patient monitoring equipment
- Diagnostic instrument front ends
- Advantage: Low drift characteristics ensure measurement stability
### Practical Advantages and Limitations
Advantages
- High common-mode voltage range: -2V to +65V with ±5V supplies
- Excellent CMRR: 80dB minimum at DC, 70dB at 10kHz
- Low offset voltage: 100μV maximum
- Low offset drift: 1.5μV/°C maximum
- Single-supply operation: +3V to +36V
- Dual-supply operation: ±1.5V to ±18V
Limitations
- Limited bandwidth: 500kHz typical
- Not suitable for RF or high-frequency applications
- Requires external gain-setting resistors
- Higher power consumption than modern CMOS alternatives
## 2. Design Considerations
### Common Design Pitfalls and Solutions
Power Supply Decoupling
- Pitfall : Inadequate decoupling causing oscillation or reduced CMRR
- Solution : Use 0.1μF ceramic capacitor close to each supply pin with 10μF bulk capacitor
Gain Resistor Selection
- Pitfall : Using standard 1% resistors causing gain error and reduced CMRR
- Solution : Use 0.1% tolerance metal film resistors or better, with matched temperature coefficients
Input Protection
- Pitfall : Input overvoltage damage in industrial environments
- Solution : Implement series resistors and clamping diodes for inputs exceeding supply rails
### Compatibility Issues
Digital Systems Interface
- The AD820B's rail-to-rail output may not reach full supply rails when driving ADC reference voltages
- Solution: Use level-shifting circuits or select ADCs with input ranges compatible with amplifier output swing
Mixed-Signal Systems
- Potential ground loop issues when interfacing with digital circuits
- Recommendation: Use separate analog and digital grounds with single-point connection
Sensor Compatibility
- Excellent compatibility with most bridge sensors and current shunts
- Caution required with high-impedance sensors (>100kΩ) due to input bias current (500nA maximum)
### PCB Layout Recommendations
Component Placement
- Place gain-setting resistors close to amplifier pins
- Position decoupling capacitors within 5mm of supply pins
- Keep feedback paths short and symmetric
Routing Guidelines
- Use ground plane for improved noise immunity
- Route
